Cerium Compounds Coating as a Single Self-Healing Layer for Corrosion Inhibition on Aluminum 3003

Abstract

The formation of cerium hydroxide was studied, and its capacity as a corrosion inhibitor on aluminum substrates was evaluated. These particles were deposited by immersing the substrate in a bath with cerium nitrate and hydrogen peroxide. Four different immersion times were used to determine the differences in behavior from low concentrations to an excess of particles on the surface. The coatings were analyzed morphologically by scanning electron microscope (SEM) and optical microscope, and chemically by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Electrochemical corrosion analysis was studied using cyclic potentiodynamic polarization (CPP), electrochemical impedance spectroscopy (EIS), and electrochemical noise (EN). The results show that for 2 and 5 min of immersion, there was corrosion inhibition caused by the presence of cerium Ce³⁺ in the coating, but with excess cerium hydroxide particles, corrosion was favored. The presence of cerium particles favors corrosion at 30 s of immersion. This is the same case at 60 min, where corrosion was favored by the excess of Ce⁴⁺ particles on the surface.

1. Introduction

New challenges are posed by the broader use of lightweight metals, especially Al and Mg and their alloys, as structural materials that present considerable prospects for different applications, such as transport or construction [1,2,3,4]. This enormous potential continues to stimulate research in corrosion prevention and the improvement of anticorrosion treatments. Various industries have a high interest in developing materials with higher corrosion resistance, such as reinforcements in construction [5,6,7,8], automotive [9,10,11,12,13,14], aeronautics [15,16,17], steel [18,19,20], and biomedical implants [21,22].

The losses to the global economy caused by corrosion have been estimated for decades and are increasing consistently [23,24]. In its latest study, ¨IMPAC¨, NACE INTERNATIONAL estimates the annual cost of corrosion at USD 2.5 trillion. It also estimates that applying the currently available anti-corrosion measures could save between 15 and 35% of the cost of corrosion for the global economy [24].

The use of protective coatings is the most-applied strategy for corrosion protection and has seen a steady evolution. Currently, the methods include such diverse approaches as aluminum anodizing [25,26,27,28,29], electrodeposition [30], sputtering [31], sol–gel deposition [32], electrophoretic deposition [33], physical vapor deposition (PVD) [34], and corrosion inhibitors [35]. Corrosion inhibitors are compounds capable of inhibiting or reducing the corrosion rate when present in small amounts. Among the corrosion inhibitors, salts of rare earth elements, specifically lanthanides, have been identified as an environmentally friendly method and have been previously reported [36,37,38,39,40,41,42]. Hinton (1984) [43] was the first who concluded that rare earth salts act as a cathodic inhibitor on an aluminum substrate, similar to that of Zn²⁺ in steel.

Among salts of rare earth elements, cerium stands out because of its natural abundance; since it is the most abundant rare earth element, it is present in amounts similar to such common elements as copper, and its extraction is easy and economically viable [44,45]. In aggressive solutions, the electrolyte tends to become more alkaline near the surface due to the oxygen reduction reaction:

$${\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-} \to 4{\mathrm{OH}}^{-}$$

and the hydrogen evolution:

$$2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-} \to { \mathrm{H}}_2+2{\mathrm{OH}}^{-}$$

Under these alkaline conditions, aluminum oxide is soluble, while cerium oxide remains insoluble. Thus, the cerium oxide protects the metal by stopping the reduction reaction, preventing the pH from increasing to values where aluminum is soluble. In addition, cerium salts have presented the best results due to the ease of Ce³⁺ to react on the metal surface.

Yasakau et al. [42] published a review of rare earth salts as healing agents and defined self-healing materials as those that can repair themselves and recover functionality after degradation, damage, and failure. Coatings that are modified with cerium particles can acquire a self-healing capacity. This active corrosion protection occurs until pores or failure sites are filled or blocked. The effectiveness of self-healing is related to the pH at the interface, concentration of cerium, and microstructural characteristics. The oxides and hydroxides formed are insoluble and can block pitting and coating failures. In this way, its expansion is prevented, the coating is self-repaired, and the corrosion rate is diminished. The increased attention to their use arises from the pressing need for alternatives to chromium compounds, one of the most widely used corrosion inhibitors, which involves a significant environmental and health risk [46,47]. Cerium-based coatings have been deposited by different methods, mainly to take advantage of its reactivity in fuel cells encapsulating nanoparticles [48], catalytic oxidation by hydrothermal method [49], corrosion by immersion in a bath [50], solar cells by solvothermal method [51], and chemical vapor deposition (CVD) [52].

This work presents the study of cerium coatings as a single layer on aluminum. The aim is to evaluate the electrochemical behavior of the species deposited at different immersion times. Cerium species deposited at short times can function as corrosion inhibitors since they are not completely oxidized. However, at long times, deposition bath conditions oxidize Ce³⁺ to Ce⁴⁺.

2. Experimental Methods and Materials

The substrate was aluminum 3003 (Al 96.8–99 wt%, Cu 0.05–0.2 wt%, Fe 0.7 wt% max, Mn 1–1.5 wt%, Si 0.6 wt% max, Zn 0.1 wt% max, residuals each 0.05 wt% max, total 0.15 wt% max) [53], and was washed with deionized water and acetone to remove impurities. All reagents used are reactive grade.

2.1. Cerium Treatment by Immersion

The immersion bath was conformed with Ce(NO₃)₃ 6H₂O (Aldrich 99%), with a concentration of 50 mM, which was added to 10% by volume of H₂O₂ (J.T. Baker 30%) to accelerate the deposition reaction of the cerium compounds. The bath was used at a temperature of 50 °C. The treatment was carried out by immersion of the substrates for 0.5, 2, 5, and 60 min of immersion; after the immersion, the substrate was washed with deionized water.

2.2. Electrochemical Tests

Electrochemical measurements were taken in a Bio-Logic potentiostat-galvanostat, Sp-300 model, using a corrosion cell and a Faraday cage. A graphite counter electrode, Ag reference electrode || AgCl (3.5 M KCl), 375 mL of 3.5% NaCl, and bubbling N₂ was used to remove dissolved oxygen. For all the tests, the samples were immersed in the solution for 1 h. Cyclic potentiodynamic polarization (CPP) was performed with a velocity of 10 mV/s from E_i (initial potential) = −0.3 V vs. OCP (open circuit potential) to 0.6 V vs. OCP, and the E_f (final potential) was 0 V vs. E_i. Tafel slopes were analyzed by extrapolating the anodic and cathodic slopes to obtain the corrosion current (I_corr) and corrosion potential (E_corr). The corrosion rate was determined by Equation (1)

$$\mathrm{CR}=\mathrm{K} \frac{{\mathrm{I}}_{\mathrm{corr} }\mathrm{EW}}{\mathsf{\rho } \mathrm{A}}$$

(1)

where CR is the corrosion rate in mm/year, K is 3272 mm/cm per year, I_corr is the corrosion current (A), and A the sample area (cm²), which is 2 cm² for the experimental cell. For AA3003, the equivalent weight (EW) is 9.07 (g/eq) [54] and the density ($\rho$) is 2.73 g/cm³ [53].

Electrochemical impedance spectroscopy measurements were performed at open circuit potential (OCP), with an amplitude of 10 mV and a frequency of 10⁵ to 1 Hz. From the EIS measurements, corrosion inhibition efficiency (IE) was calculated using Equation (2).

$$\mathrm{IE}=\left(1-\frac{{\mathrm{R}}_{\mathrm{ct}}^0}{{\mathrm{R}}_{\mathrm{ct}}}\right) · 100$$

(2)

where IE is the inhibition efficiency in % and ${\mathrm{R}}_{\mathrm{ct}}^0$ and R_ct are the resistance to the transfer of untreated aluminum charge and the sample (Ω), respectively [55].

Electrochemical noise measurements were taken by measuring the potential between two nominally identical electrodes, the substrate. The current measurement between two equal working electrodes, with a negligible potential difference or zero resistance ammeter (ZRA), was performed for 15 min without applying external potential. A second-degree polynomial adjustment was used to remove the effect of the direct current component. The standard deviation of potential (σ_E) and current (σ_I) was calculated. The noise resistance (Rn) was calculated by Equation (3).

$${\mathrm{R}}_{\mathrm{n}}=\left(\frac{{\mathsf{\sigma }}_{\mathrm{E}}}{{\mathsf{\sigma }}_{\mathrm{I}}}\right)$$

(3)

where R_n is the noise resistance in Ω, σ_E is the standard deviation of the potential in V, and σ_I is the standard deviation of the current (A).

The pitting index (PI) was calculated by Equation (4).

$$\mathrm{PI}=\left(\frac{{\mathsf{\sigma }}_{\mathrm{I}}}{{\mathrm{I}}_{\mathrm{RMS}}} \right)$$

(4)

where PI is the pitting index, and I_RMS is the root mean square of the current (A) [55].

2.3. Characterization

The surface inspection was carried out in a digital microscope, the Keyence VHX-5000 series. The depth and profile measurements were taken with the VHX software of the microscope. Scanning electronic microscopy (SEM) micrographs, energy dispersive X-ray spectroscopy (EDS) microanalysis, and elemental mapping were performed with a JEOL model JSM-6510 scanning electron microscope with built-in EDS. The X-ray photoelectron spectroscopy (XPS) was elaborated with a Thermo Scientific K-Alpha™+ spectrometer, which used an Al Kα monochromatized X-ray source (hν = 1486.6 eV). The operating pressure was approximately 10⁻⁹ mBar, with a spot size of 400 μm, step energy of 20.0 eV, with a total of 10 scans, and an erosion of 15 s to reduce adsorbed atmospheric compounds. All acquired spectra were processed using a Gaussian peak type and the references are reported from NIST Standard Reference Database 20, Version 4.1, and references of reported characterizations by XPS of cerium [56,57,58,59] and O1s spectra [60,61]. Great care was put into the baseline and deconvolution assignations for the XPS spectra to avoid misinterpretations [62].

3. Results and Discussion

Figure 1 shows the physical appearance of the samples treated with the different immersion times, and the aluminum 3003 without treatment is presented as a reference. There were no significant changes in the surface appearance in the first three times. In the case of the sample treated for 1 h, a complete change in the surface was observed, and a coating in a beige tone was appreciated, which was the first indicator of long immersion times favoring the formation of CeO₂.

SEM analyses were conducted to observe the particles formed on the surface. Figure 2 shows the growth of particles on immersed surfaces. The SEM micrograph of Figure 2c shows that the formation of the particles began at short immersion times since, at 0.5 min, their presence was already observed, although in a very localized way. The rapid formation of the particles can be attributed to the addition of hydrogen peroxide in the cerium nitrate solution.

The number of agglomerates and the height of these were considerably increased at 2 min of immersion (Figure 2e). In addition, the growth occurred around the first agglomerates and not homogeneously throughout the substrate.

Figure 2g shows that the particles covered the entire surface after 5 min, and areas with a higher number of particles were observed. Figure 2i shows that, at 60 min of immersion, a coating was formed with a higher thickness than that found at 5 min. Once the initial particles covered the entire surface, the newly deposited material started to grow particles on top of the first layer, increasing the thickness.

Figure 2b,d,f,h,j show the elemental mapping by EDS progressively with the immersion time. The quantity of blue zones attributed to cerium was increasing accordingly to the immersion time. Isolated areas of cerium were observed at the beginning. Finally, at 60 min, the areas with the presence of cerium were predominant.

Table 1. EDS analysis at different immersion times.

Time (min)	Al (wt%)	C (wt%)	O (wt%)	Mn (wt%)	Ce (wt%)
0	92.07	5.58	1.15	1.20	-
0.5	87.82	9.07	2.13	0.77	0.22
2	84.07	9.84	2.88	1.46	1.75
5	86.37	6.77	3.67	1.36	1.84
60	63.20	9.06	10.22	-	17.52

shows the surface composition, where aluminum was the predominant element. The cerium at 0.5 min had a concentration of 0.22%, at 2 min 1.75%, at 5 min 1.84%, and at 60 min, the concentration of cerium on the surface reached 17.52%. In the composition analyses, no nitrogen was found, which could be present because of the cerium salt.

XPS analyses shown in Figure 3 confirmed the presence of Ce³⁺ and Ce⁴⁺. It can be divided into two spin orbits, 3d 3/2 and 3d 5/2 orbitals. In the Ce3d scan, the signals found in 904 eV, 903 eV, 885 eV, and 880 eV were attributed to the Ce³⁺ species. The signals in 916 eV, 907 eV, 906 eV, 900 eV, 898 eV, 888 eV, and 882 eV were attributed to the Ce⁴⁺ species. The presence of OH⁻ was shown in the signals 531 eV and 530 eV. The O²⁻ corresponded to the 529 eV signal. The Ce³⁺ peaks were attributed to the presence of Ce₂O₃ and Ce(OH)₃. At the same time, the Ce⁴⁺ signal was attributed to CeO₂.

Table 2. Atomic % of Ce species at 2 and 60 min of immersion quantified using XPS.

Time (min)	Ce3d Scan	Atomic %	O1s Scan	Atomic %
2 min	Ce3+	72.45154	OH−	100
	Ce4+	27.54846	O2−	-
60 min	Ce3+	43.50885	OH−	82.41873
	Ce4+	66.49118	O2−	17.58127

shows the atomic percentage of each oxidation state in the coating at 2 min and 60 min of immersion. In the case of 2 min of immersion, Ce³⁺ and OH⁻ were in higher concentration, which indicates that the deposit was formed mainly by Ce(OH)₃, with a low concentration of Ce₂O₃ and CeO₂. For the sample with the immersion of 60 min, the atomic percentage of Ce⁴⁺ increased, and a peak of O²⁻ appeared, so the formation of CeO₂ was favored at long immersion times due to the oxidation of Ce(OH)₃ and Ce₂O₃.

Electrochemical behavior was evaluated to determine the degree to which cerium hydroxide modified the corrosion process on the surface. Figure 4 shows the cyclic potentiodynamic polarization (CPP) response. The samples of 0, 0.5, 5, and 60 min had the potential for initiating and propagating pits (E_pit) equal to the corrosion potential (E_corr).

This is because the increase in anodic current occurs without variations in the slope. In comparison, the 2 min sample had a variation of 1 mV between the E_corr and the E_pit. This is observed in the slight slope change of the anodic part, indicating that the pits began to spread to a more positive potential in that sample. Hysteresis cycles were formed in all samples by reversing the potential. This is attributed to the fact that, once the pits started, they spread to even more negative potentials. The potential in which hysteresis closed was attributed to the pits ceasing to spread and the aluminum becoming passive.

Table 3. Values of the CPP variables for aluminum 3003 at different immersion times in the cerium bath.

Time (min)	Ecorr (mV)	Epit (mV)	Icorr (μA/cm2)	Corrosion Rate (mmpy)
0	−734.971 ± 22.39	Ecorr = Epit	0.304 ± 0.05	0.00164 ± 0.0002
0.5	−753.144 ± 32.52	Ecorr = Epit	0.314 ± 0.34	0.00074 ± 0.0003
2	−805.145 ± 32.49	−690. 2 ± 27.63	0.098 ± 0.05	0.00053 ± 0.0003
5	−747.411 ± 33.89	Ecorr = Epit	0.140 ± 0.10	0.00076 ± 0.0005
60	−676.997 ± 45.95	Ecorr = Epit	1.471 ± 0.59	0.02262 ± 0.0196

shows the results of the corrosion rate. It is shown that aluminum 3003 had a corrosion rate of 1.64 × 10⁻³ mmpy. At 0.5, 2, and 5 min of immersion, the corrosion rate decreased by one order of magnitude. However, in the sample at 60 min, the corrosion occurred faster due to cerium excess. In the corrosion current, it was observed that the sample that presented the lowest current demand was the 2 min one, so it was the sample that presented less corrosion. The sample of 60 min of immersion had the highest I_corr. This is because the combined response of cerium hydroxide oxidation and aluminum corrosion increased the corrosion current.

Figure 5 shows that the Nyquist diagram can be approximated to be a semicircle. The difference in the diameter of that semicircle can be attributed to the difference in roughness and an inhomogeneous surface that caused frequency dispersion. Using ZView 2 software, simulations were performed to obtain the equivalent circuit of the system. The obtained result is a system composed of the resistance of the electrolyte (Rs), resistance-to-charge transfer (Rct), which is the value directly related to the corrosion of the system, and a constant phase element (CPE) that is related to the capacitance of the double layer. The results of corrosion inhibition efficiency are shown in

Table 4. Results of EIS measurements.

Time (min)	Rs (Ω)	CPE		Rct (Ω)	% Inhibition
		T	P
0	7.10 ± 1.37	5.99 × 10−6 ± 1.28 × 10−6	0.91 ± 0.007	52,712.67 ± 4303	-
0.5	6.96 ± 0.26	8.97 × 10−6 ± 3.27 × 10−6	0.93 ± 0.01	34,384.5 ± 5197.94	−53.30
2	8.10 ± 1.73	3.77 × 10−6 ± 5.21 × 10−6	0.47 ± 0.6	98,687 ± 38,895	46.58
5	7.84 ± 0.91	8.68 × 10−6 ± 2.73 × 10−6	0.94 ± 0.02	54,616 ± 4971.58	3.84
60	16.58 ± 10.69	2.15 × 10−5 ± 4.5 × 10−6	0.97 ± 0.004	5850.33 ± 1700.79	−801.02

.

The spectrum at 0 min represents aluminum without treatment. The spectra of the samples with 0.5 and 60 min of immersion show a smaller semicircle, which indicates that there was a higher charge transfer and, therefore, higher corrosion. In the case of the spectra of 2 and 5 min, larger semicircles are observed than that of 0 min. This indicates a higher difficulty for the flow of electrons in this sample, so there was less corrosion.

The sample that presented the best corrosion inhibition is that of 2 min, which increased the resistance-to-charge transfer by 46.58%. At 5 min, the efficiency dropped considerably to 3.84%. In the samples of 0.5 and 60 min, there was a negative inhibition efficiency, which indicates that, in these samples, there was higher corrosion than in untreated aluminum. Hence, the presence of cerium favored corrosion. In the sample at 60 min, it reached a resistance-to-charge transfer of approximately 801.02% lower than the untreated aluminum.

Electrochemical noise measurements are shown in Figure 6. Due to the absence of an external potential or current, these variations in current and potential were related to changes in the speed of anodic and cathodic reactions, rupture, and failures in the continuity of the surface that cause pit formation. At 0, 0.5, 2, and 5 min of immersion, the current oscillated around 0.1 μA. In comparison, the sample at 60 min had the highest current with variations around 1.5 μA. This higher demand for current was due to higher activity on the surface produced by a higher corrosion rate and increased surface area caused by pitting.

Table 5. Electrochemical noise results.

Time (min)	σE	σI	Rn (Ω)	Ip	Type of Corrosion
0	0.00186	2762 × 10−8	82,407.23 ± 14,417	0.58± 0.6	Localized
0.5	0.00152	3.29 × 10−8	46,101.33 ± 5252.19	0.6 ± 0.4	Localized
2	0.00130	1379 × 10−8	112,155.48 ± 26,074	0.49 ± 0.2	Localized
5	0.00177	3.85 × 10−8	31,491 ± 7942.50	0.3 ± 0.04	Localized
60	0.00080	17,547 × 10−7	6446.98 ± 5083	0.2 ± 0.06	Localized

shows the results of the pitting index and noise resistance. The results of the Rn show that the 2 min of immersion gave higher resistance to corrosion. The Rn at 2 min was 112,155 Ω. This represents a higher Rn than the untreated aluminum sample, indicating a slower corrosion rate and higher resistance-to-charge transfer. However, in the sample at 60 min, the Rn value dropped considerably to 6446 Ω and shows that it was more prone to corrosion. Pitting index values between 0.1 and 1 indicate localized corrosion, values between 0.1 and 0.01 indicate mixed corrosion, and values between 0.01 and 0.001 indicate widespread corrosion. The obtained results indicated localized corrosion at all the tested times.

Two samples were analyzed after corrosion to expound the electrochemical behavior, the longer (60 min) and shorter (2 min) testing times. The microscope images in Figure 7a,d,g show that untreated aluminum had several pits (Figure 7a,b). By analyzing the surface profile (Figure 7g), it was determined that the pits had a depth of approximately 4 μm. The images in Figure 7b,e,h visually show these areas where there was electrochemical activity. The sample at 2 min of immersion presented a lower number of pits and its depth decreased to approximately 1 μm.

In the case of 60 min of immersion, Figure 7c,f,i show that in cerium excess conditions, there was the most significant damage caused by corrosion. The 3D image (Figure 7f) and the surface profile (Figure 7i) show that the pitting increased its depth considerably, reaching 25 μm. This higher electrochemical activity was associated with the most intense localized corrosion favored by the cerium excess.

XPS analyses were conducted after corrosion to identify the changes in the chemical composition. Figure 8a–c and

Table 6. XPS analyses of coating at 2 and 60 min of immersion after corrosion.

Time (min)	Ce3d Scan	Atomic %	O1s Scan	Atomic %
2 min	Ce3+	54.66105	OH−	100
	Ce4+	45.33894	O2−	-
60 min	Ce3+	39.04718	OH−	100
	Ce4+	60.95282	O2−	-

show the analyses of XPS for 2 min of immersion after corrosion. In Figure 8a, the area studied is indicated by a red oval. The XPS analysis of the Ce3d and O1s scans shows that the atomic percentage of Ce³⁺ decreased by approximately 20% compared with the coating before corrosion. This is because, during corrosion, there was an increase in pH at the substrate–electrolyte interface caused by the cathodic reaction. This process resulted in the formation of CeO₂, which was insoluble under these conditions and blocked the cathodic sites where pitting began, thus reducing corrosion. The process is shown below and has been reported by different authors [56,57,63,64].

The corrosion process occurred in two semi-reactions.

• Anodic reaction

$$3\mathrm{Al} \to { \mathrm{Al}}^{3+}+3{\mathrm{e}}^{-}$$

2.

Cathodic reaction

$${\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-} \to 4{\mathrm{OH}}^{-}$$

3.

Active site blocking

Because of the product of the cathodic reaction, cerium(III) hydroxide was oxidized. This reaction occurred in cathodic sites where the corrosion process was happening.

$$2\mathrm{Ce}{\left(\mathrm{OH}\right)}_3+2{\mathrm{OH}}^{-}\to 2{\mathrm{CeO}}_2+2{\mathrm{e}}^{-}+4{\mathrm{H}}_2$$

The final product of the process was CeO₂, which was stable in conditions where corrosion occurred and favored the blocking of cathodic sites. This result explains the electrochemical behavior of the samples with a low concentration of cerium species.

In the case of the sample with a high concentration of cerium, Figure 8d,e,f show the analyses after corrosion. Figure 8d shows the area where the analysis of the corrosion products was carried out. The high-resolution XPS spectrum of Ce3d and O1s in Figure 8e,f and Table 6 show no significant difference in the atomic percentage of the Ce³⁺ species before and after corrosion. This is because most of the cerium(III) hydroxide/oxide that was available to be oxidized was, in fact, oxidized during the deposition bath due to the long treatment time.

4. Conclusions

The coatings of cerium species by immersion in a bath with H₂O₂ are formed in short times by Ce³⁺ species, mainly Ce(OH)₃ and Ce₂O₃ in lower concentrations, making them an excellent option for application as a corrosion inhibitor. At long times, the percentage of these Ce³⁺ species decreases, and they are mostly oxidized to CeO₂, which means that long immersion times are not viable as corrosion inhibitors. Elemental analysis shows that the cerium species are homogeneously distributed in the coating. The electrochemical behavior indicates that there is susceptibility to pitting corrosion. At short immersion times, the presence of cerium slightly favors corrosion, while at long times, corrosion is significantly favored. The best result was obtained at 2 min of immersion, where electrochemical tests show that corrosion was inhibited by up to 46%.

It can be concluded that the use of cerium concentrations below 1% and in excess favor corrosion. The post-corrosion analysis shows that, on low concentration coatings, the oxidation reaction of the Ce³⁺ to Ce⁴⁺ species takes place. Inhibition is attributed to blocking cathode sites by cerium hydroxide particles, which oxidize to cerium oxide and stop the spread of pitting. Therefore, using a coating as a protective barrier would improve the protection against corrosion, as obtained in this work.